The present invention relates to an exposure technology and, more particularly, to a technology which is effective if applied to the photolithography process of a semiconductor integrated circuit device.
If the high integration of the semiconductor integrated circuit advances so that the circuit elements and the wiring design rules come to the sub-micron order, the photolithography process for transferring a circuit pattern on a mask onto a semiconductor wafer by making use of a beam of g- or i-line is troubled by a serious problem of a reduction in the precision of the circuit pattern to be transferred onto the wafer. In case, for example, a circuit pattern formed of transparent regions P1 and P2 and a shielding region N over a mask 20, as shown in FIG. 1J-1, is to be transferred onto the wafer, the phases of the lights L just after having passed through the paired transparent regions P1 and P2 interposing the shielding region N are in phase with each other, as shown in FIG. 1J-2. As a result, the two lights interfere with each other at that portion on the wafer, which might otherwise be intrinsically the shielding region, so that they are intensified (as shown in FIG. 1J-3). As a result, the contrast of the projected image on the wafer drops with a reduced focal depth, as shown in FIG. 1J-4, so that the pattern transfer precision is seriously degraded.
As means for solving this problem, there has been proposed a phase shift technology for preventing the drop of the contrast of the projected image by changing the phases of the lights to transmit the mask. In Japanese Patent Publication No. 59296/1987, for example, there is disclosed a phase shifting technology, in which one of the paired transparent regions across the shielding region is formed with a transparent film to establish a phase difference between the lights having passed through the two transparent regions at the time of exposure so that the interfering lights may be weakened at that portion on the wafer, which might otherwise be the shielding region. When a circuit pattern formed on a mask 21, as shown in FIG. 1K-1, is to be transferred onto a wafer, either of the-paired transparent regions P1 and P2 interposing the shielding region N in between is formed with a transparent film 22 having a predetermined refractive index. By adjusting the thickness of the transparent film 22, moreover, the individual lights having passed through the transparent regions P1 and P2 go out of phase of 180 degrees, as shown in FIG. 1K-2, so that they interfere with each other in the shielding region N on the wafer and are weakened (as shown in FIG. 1K-3). As a result, the contrast of the projected image on the wafer is improved, as shown in FIG. 1K-4, to improve the resolution and the focal depth and accordingly the transfer precision of the circuit pattern formed on the mask 21.
In Japanese Patent Laid-Open No. 67514/1987, on the other hand, there is disclosed a phase shift technology, in which a phase difference is established between a light having passed through a transparent region and a light having passed through a fine opening pattern, by removing the shielding region of a mask partially to form the opening pattern and by forming a transparent film in either the opening pattern or the transparent region existing in the vicinity of the opening pattern, so that the light having passed through the transparent region may be prevented from having its amplitude distribution expanded transversely.
The phase shifting method, by which one mask is formed thereon with an ordinary pattern (or main pattern) and a shifter pattern (or accompanying or complementary pattern) for giving a phase opposed to that of the former, will be called hereinafter the xe2x80x9con-mask phase shifting methodxe2x80x9d and will be called the xe2x80x9con-mask phase inversion shifting methodxe2x80x9d especially in case the phase shift is (2n+1)xcfx80 (wherein n: an integer).
In Japanese Patent Laid-Open No. 109228/1985, moreover, there is disclosed a method, in which two masks are simultaneously illuminated to improve the throughput of a projecting exposure so that the portions of one wafer corresponding to different chips may be simultaneously exposed. In Japanese Patent Laid-Open No. 107835/1985, on the other hand, there is disclosed a technology, in which two masks having an identical pattern can be exposed without any trouble even if one of them is defective, by dividing one exposing line into two halves to illuminate the identical portion of the two masks and by composing them to expose the wafer.
However, these two disclosures are not effective in the least for improving the resolution although they are effective for preventing the defect on the mask pattern from being transferred onto the wafer or for improving the throughput.
According to our examinations, the aforementioned phase shifting technology of the prior art, in which the transmitting region of the mask is in its portion with the transparent film so that a phase difference may be established between the light having passed therethrough and the light having passed through the neighborhood transmitting region, is troubled by a problem that the manufacture of the mask takes a long time period and many works.
Specifically, the actual mask formed with an integrated circuit pattern is complicated by various patterns so that the mask makes it seriously difficult to select the place to be arranged with the transparent film thereby to exert serious restrictions upon the pattern design. In case the mask is formed with the transparent film, on the other hand, in addition to the step of inspecting the existence of a defect in the integrated circuit pattern, there is required a step of inspecting the existence of a defect in the transparent film so that the mask inspecting step is seriously complicated. In case, moreover, the mask is formed with the transparent film, foreign substances to be caught by the mask are increased to make it difficult to prepare a clean mask.
An object of the present invention is to provide a phase shifting technology which has succeeded in solving the above-specified problems.
Another object of the present invention is to provide a size-reducing projection exposure technology which is enabled to give the best image plane to the individual planes to be exposed, if stepped, by a single exposure.
A further object of the present invention is to provide a projection exposure technology which can extend the exposure limit of a fine pattern by violet or ultraviolet rays to a finer range.
A further object of the present invention is to provide a projection exposure technology which can compose and expose two master patterns.
A further object of the present invention is to provide a size-reducing projection exposure technology which can compose and interfere two mask patterns to be projected and exposed, even in case an interference distance of a light source is short.
A further object of the present invention is to provide a mask pattern layout technology which is useful for fabricating an integrated circuit by using the phase shifting method.
A further object of the present invention is to provide a projection exposure technology which is useful for fabricating an SRAM by using the phase shifting method or the like.
A further object of the present invention is to provide an exposure technology which is useful for fabricating a highly integrated semiconductor circuit such as a DRAM having a fine size as small as the exposure wavelength.
A further object of the present invention is to provide a projection exposure technology which is effective for exposing a periodic fine pattern.
A further object of the present invention is to provide a projection exposure technology which is effective if applied to an excimer laser exposure technology.
A further object of the present invention is to provide a mask inspection technology which is useful for inspecting a mask to be used in the phase shifting method.
The invention to be disclosed hereinafter will be briefly described in the following in connection with its representatives.
According to one representative of the present invention, there is provided the following exposure method. When a predetermined pattern, which is formed on a mask and composed of a shielding region and a transparent region, is to be transferred onto a specimen to be illuminated, by irradiating the mask with a light and irradiating the specimen with the light having passed through the transparent region of said mask, the light emitted from a light source is divided into two lights, and the phases of the two lights immediately after having passed through different portions of said mask are opposed to each other by changing the individual optical lengths for said two lights to reach said mask. After this, said two lights are composed to illuminate said specimen.
According to the above-specified means, the two lights immediately after having passed through the different portions of the mask have their phases opposed to each other and are then composed to illuminate the specimen. As a result, one light having passed through a predetermined transparent region on the mask and the other light having passed through another transparent region on the mask interfere with each other and are weakened in their boundary regions at the portions, in which they are arranged close to each other, so that the projected image has its contrast improved drastically.